The invention relates to a reduction gear for the starter-generator of an internal combustion engine, the starter-generator being drive-connected to its crankshaft via a belt drive and the reduction gear being a planetary gear train arranged in the transmission path between the crankshaft and a first belt pulley coaxial with the crankshaft.
The effort to reduce the cost and weight of motor vehicles and their engines extends to the auxiliaries, particularly where these are heavy and expensive electrical machines that are never in operation simultaneously. A starter-generator connected to the crankshaft via a belt drive was already being used in small cars (PUGH 500) some decades ago. In larger, high-compression engines a high torque with lower speed is needed for starting, whereas generator operation requires a high speed. A multistep reduction gear is therefore needed between crankshaft and starter-generator.
The Report on the Technical Symposium xe2x80x9cElectrical Machine in the Power Trainxe2x80x9d of Sep. 4, 1999, organized by the Technical Universities of Chemnitz and Graz, discloses the use of a two-stage planetary gear train, which is arranged either coaxially with the crankshaft, preferably in the belt pulley, or coaxially with the starter-generator. In this it is proposed that the change-over should occur automatically due to the helical toothing of the planetary gear train as a function of the torque flow direction. Apart from the sluggishness of the change-over, however, this does not permit control of all possible engine operating conditions and their transitional states. One only has to think of rapidly revving up the engine after successful starting, of rapid backing-off of the throttle from high engine speed, or backfiring in the event of mistimed ignition. It is moreover desirable to be able to interrupt the connection via the reduction gear in certain operating conditions.
The patent specification U.S. Pat. No. 5,132,604 likewise discloses the arrangement of a planetary gear train as reduction gear for a starter-generator on the crankshaft axis. In this an overrunning clutch, via which the entire power output is therefore transmitted, is already inserted between planet carrier and output shaft. This calls for very large overrunning clutches, which are correspondingly slow to switch. Planetary gear trains, for their part, present problems, since their geometry makes the desired optimum reduction of between 0.5 and 0.2 for the same directions of rotation in both gears unfeasible. In addition, the costs and weight of planetary gear trains and the overall space taken up by these are still considerable. The latter leads to very large belt pulleys where the planetary gear train is to be enclosed by one.
WO-OS 99/23398 discloses a multistage manual shift transmission having multiple planetary sets arranged in series with a xe2x80x9ccoplanarxe2x80x9d planet wheel. In this the gears are changed by means of hydraulically actuated friction clutches, or band brakes and overrunning clutches.
The object of the invention is to design a reduction gear of the type outlined in the introductory part, so that the required gear ratio is achieved with reduced costs and dimensions and can be changed so that it is present in all conceivable operating conditions. According to the invention this is achieved by the following design features:
a) The planetary gear train has a housing and a planetary gear stage comprising an internal gear, a sun gear, a cage and a planetary ring with internal teeth and external teeth,
b) the internal gear meshes with the external teeth of the planetary ring and is drive-connected to the crankshaft,
c) the sun gear meshes with the internal teeth of the planetary ring and can be rotationally locked to the housing by means of a first clutch, corresponding to the starter position,
d) the cage has eccentric bearings for the planetary ring, is rotationally locked to the first belt pulley, and is rotationally locked by means of a second clutch to one of the members of the planetary gear train, so that the planetary gear train rotates as a whole, corresponding to the generator position.
Planetary gear trains according to feature a) are cheaper and lighter than conventional planetary gear trains and take up less overall space. This applies in particular to the increment between direct and geared running in the same direction of rotation required in the application forming the subject matter. This allows them to be fitted inside the belt pulley. In gears for driving the wheels of motor vehicles, the increments are significantly smaller.
These advantages are further enhanced by the attachment of the members of this planetary gear train according to b) and c). Despite the high torque on the large radius of the internal gear required for starting, the drive connection of internal gear and crankshaft only results in small tooth forces. The sun gear serves merely as reaction member, being beneficial to the clutch connection. Coupling to the fixed housing creates the gear ratio for starting; in operation of the generator it is not under load.
Combining cage and belt pulley in accordance with d) enables the bearing forces produced by the belt tension to be neatly absorbed. The second clutch can also readily be connected between the cage and another member of the planetary gear train. This means that in generator operation the entire gear mechanism rotates as a whole, the losses being minimal. As for the clutches, virtually all known types are in principle feasible: friction disk clutches, positively interlocking clutches with claws or teeth, band or wrap spring clutches, electromagnetic clutches, overrunning clutches with roller or clamping elements, centrifugal clutches being a possible alternative or addition.
In a preferred and particularly simple design, the internal gear is rotation-ally locked to the crankshaft and the cage can be rotationally locked to the internal gear by means of a second clutch. This is the most common case with a single transmission stage for engine starting. In a first embodiment the first and second clutch are overrunning clutches, the first clutch closing when the cage is being driven, and the second clutch closing when the cage is driving. First and second clutches are therefore fitted in opposition. This therefore provides direct drive in generator operation and an optimum gear ratio for starting. As soon as the engine begins to run up after starting, the change-over in the generator occurs rapidly and automatically but nevertheless smoothly. therefore fitted in opposition. This therefore provides direct drive in generator operation and an optimum gear ratio for starting. As soon as the engine begins to run up after starting, the change-over in the generator occurs rapidly and automatically but nevertheless smoothly.
In one development, the two clutches are remote-controlled clutches, the first clutch being closed for starting of the internal combustion engine and the second clutch being closed in normal generator operation, all of the aforementioned clutches again being feasible, including remote-controlled overrunning clutches, which through actuation can be brought into the opened position, in which they remain even if the torque direction is reversed. As a result, closing of the overrunning clutch cannot occur even in the event of sudden engine deceleration or misfiring. In addition, other auxiliaries driven by the belt drive can therefore be kept in operation even when the internal combustion engine is at a standstill.
The remote control may be achieved in widely varying ways, but the two clutches are preferably operated by electrical actuators, which are activated by a control as a function of operating conditions. This provides a high level of reliability even in extreme and abnormal operating conditions.
The gear constellation according to the invention also gives the control the facility, when the internal combustion engine is switched off and under certain temperature conditions, for opening both clutches and allowing the starter-generator to run as a motor. The belt drive, in fact, often also serves other auxiliaries, such as a coolant pump, a radiator fan or the compressor of an air-conditioning system. The neutral position of the gear offers the facility for driving all of these auxiliaries from the starter-generator when the engine is at a standstill. The temperature condition may be the coolant temperature or the ambient temperature, for example.
In a development of the invention, the reduction gear has a further engageable planetary gear stage comprising a further internal gear, a further sun gear, a further cage and a further planetary ring with internal teeth and external teeth, via which gear stage the internal gear can be connected to the crankshaft. The further planetary gear stage permits a further gear ratio and hence two different gear ratios for starting, for example: one for starting with the engine hot and one for cold-starting in winter. This development preferably resides in the fact that
a) The further internal gear meshes with the external teeth of the further planetary ring, is drive-connected to the crankshaft and can be connected by means of a third clutch to the internal gear.
b) The further sun gear meshes with the internal teeth of the further planetary ring and together with the sun gear can be rotationally locked to the housing by means of the first clutch.
c) The further cage has eccentric bearings for the further planetary ring, can be rotationally locked to the internal gear by means of the third clutch (cold starting), and when rotating as a whole can be rotationally locked by means of the second clutch to one of the members of the planetary gear train (generator).
When the further internal gear is connected to the internal gear (of the first planetary gear stage) in accordance with a), the second planetary gear stage is bypassed and rotates idly as a whole. This is the position for ordinary starting or hot-starting of the engine. The two sun gears both connected to the first clutch (b) make manufacturing and operation simpler; starting position in both starting gear ratios means that the first clutch is closed. When the third clutch in accordance with c) establishes the connection between the further cage and the internal gear (of the first planetary gear stage) the reduction gear is in the position for cold-starting of the engine, that is with a particularly high gear ratio.
This third clutch is a clutch having two switching positions, which is controlled by the control as a function of a temperature. As a result, in cold-starting the larger gear ratio is automatically selected, thereby facilitating breakaway of the engine without jeopardizing the power supply of a starter motor.